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Strength-Duration curve

Strength duration curve is a graph between electrical stimuli of different intensities and recording the time needed by each stimulus to start the response.

In this curve the strength of impulse of various duration required to produce contraction in a muscle is obtained by joining points, that graphically represent the S-D curve .

OPTIMUM TIMING FOR S-D CURVE- 

  • 10-14 days after injury
  • 10- 14 days taken for wallerian degeneration, when degeneration completed→ S-D graph plot.
  • After 21-22 days regeneration of nerve will start, after regeneration→changes found in s-d graph

METHOD TO OBTAIN THE S-D CURVE

  • Select a neuromuscular stimulator, that provide rectangular impulses of various duration, which may be the constant current or constant voltage type.
  • The patient must be warm and fully supported.
  • Passive electrode may be applied over some convenient area usually on the
    • Midline of the body or
    • The origin of the muscle group
  • Active electrode placed over fleshy part of muscle.( sometime two small electrode may be used, one over each end of muscle belly)
  • In this curve, the strength of the stimulus is plotted ( in volts), vertically and the duration (in millisecond) horizontally.
  • To start with, a stimulus with higher strength or voltage is applied.
  • The minimum duration taken by the stimulus with particular strength to excite the tissue is noted.
  • All the result are plotted and the curve is obtained.

At present, certain computerized machines such as PHYACTION, INTELLECT, etc. provide instant plotting of the curve with the memory for storing, so it can be compared with the previous curve on the same screen for easy interpretation.

CHARACTERISTICS FEATURES OF CURVE

1) RHEOBASE

  • Minimum strength (voltage) of stimulus, which can excite the tissue.
  • The voltage below this can not excite the tissue, whatever may be the duration of the stimulus.
  • It is required to produce a minimal palpable contraction.

FACTORS RESPONSIBLE FOR RHEOBASE-

  • SKIN RESISTANCE-
    • Dry and scaly skin, alters the value of rheobase (value increases)
  • EDEMA AND INFLAMMATION-
    • This requires a high intensity of current, making the value of rheobase increased.
  • Ischemia and pain-
    • It decreased tolerance of intensities, therefore altering the value of rheobase.
  • Temperature- 
    • Heat, decrease the value of rheobase.
    • Cold , increase the value of rheobase.

2) UTILIZATION TIME-

  • Minimum time required for a rheobasic strength of stimulus to excite the tissue.

3) CHRONAXIE-

  • Minimum time required for a stimulus with double the rheobasic strength (voltage) to excite the tissue.
  • It is necessary to induce minimal visible contraction.

FACTORS RESPONSIBLE FOR CHRONAXIE-

  • Texture of skin-
    • dry skin will alter the value of chronaxie and makes it difficult to find
  • Ischemia-
    • It raises the threshold values and decrease muscle excitability.
    • Chronaxie rises by  percent under ischemic condition.
  • Edema-
    • Difficulty in obtaining chronaxie value
  • Fatigue-
    • value of chronaxie may increase to double
  • Positioning  of stimulating electrode-
    • If the stimulating electrode is not positioned on the motor point of the nerve, than the value of the chronaxie of the muscle may be 10 times greater than the chronaxie at the motor point.

PRACTICAL USES OF S-D CURVE IN DIAGNOSIS-

  1. To detect excitable nerve fibers in the muscle.
  2. Asses the extent of the deneration/innervations.
  3. Detect the sign of reinnervation.
  4. It monitor the progress of the lesion and denotes whether the lesion is recovering / progressing.
  5. It permits the identification of the utilization time. it is the point at which the cure begins to flatten horizontally.

 

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